If you used to run you should have no problem doing it. Invest in some really good running shoes and start out slow. Add in some biking to help build endurance and take some of the pounding off your legs. There are some great running forums that will give you good training tips. Have fun!

do it! i did, and with less training than a month. i’ll just echo some of the above comments and say 1) don’t do it to win it but rather just to finish, 2) relatedly, don’t push yourself to a crazy beyond your limit (there’s no shame in taking the half slower, even walking some parts), 3) good running shoes!...they should feel like little pillows on your feet :), 4) know that you’re gonna be sore, real sore, for like a week, but that’s cool :)